A starting device of this type is known in the prior art as disclosed in Laid-Open Japanese Patent No. Sho 60-175763. Such a prior-art starting device 1, as shown in FIG. 1, is constituted of a DC motor 2, an overrunning clutch device 4 which is fitted slidably on an output rotating shaft 3, a planetary reduction gear 5 which transmits the rotation of an armature rotating shaft 2a of the DC motor 2 after speed reduction to an outer clutch 4a of the overrunning clutch device 4 through the output rotating shaft 3, and a shift lever 8 with its one end engaged with a plunger rod 6a of an electromagnetic switch device 6 disposed on the side section of the DC motor and with its other end engaged with an annular member 7 mounted on the overrunning clutch device 4 such that the overrunning clutch device 4 can slide on the output rotating shaft 3.
The planetary reduction gear 5 of the prior-art starting device 1 will be further explained. This planetary reduction gear 5 consists of a sun gear 5a which is formed integral with the outer peripheral section of one end of the armature rotating shaft 2a of the DC motor 2, an internal gear 5b fixed around this sun gear 5a positioned at center, and a plurality of planet pinions 5e which are in mesh with the sun gear 5a and the internal gear 5b and rotatably mounted on pins 5d fixed on a carrier 5c through sleeve bearings 5f. The carrier 5c on which each planet pinion 5e is mounted is formed integrally with the output rotating shaft 3, and accordingly the revolution of the planet pinions 5d directly becomes the rotation of the output rotating shaft 3.
The output rotating shaft 3 slidably supporting the overrunning clutch device 4 at a position adjacent to the planetary reduction gear 5 has helical splines 3a formed over the range of sliding of the clutch device 4 in order to transmit the rotation of the outer clutch 4a of the clutch device 4. Numeral 9 denotes a pinion formed on the inner clutch 4b of the overrunning clutch device 4. This pinion comes into engagement with a ring gear (not illustrated) of the engine when the overrunning clutch device 4 has slid on the output rotating shaft 3. As described above, the helical splines 3a formed on the output rotating shaft 3 are engaged with the outer clutch 4a of the overrunning clutch device 4 supported on this output rotation shaft 3, thereby enabling the constant transmission of a turning force to the outer clutch 4a while sliding the clutch device in the axial direction.
The prior-art starting device with a planetary reduction gear has such a problem that, as described above, the use of a longer output rotating shaft and consequently the starting device of increased overall length are required because of the formation of the helical splines on the output rotating shaft over the range of sliding of the clutch device to transmit rotation decelerated by the planetary reduction gear to the outer clutch 4a of the overrunning clutch device 4.